The invention relates to a method for making a handover in a CDMA cellular radio system, which comprises in each cell at least one base station connected to the mobile stations in the cell, this connection comprising both a traffic channel and one or more control channels, and in which the transmissions of the base stations are not synchronized with each other, in which method a mobile station, after having established a simultaneous connection with two or more base stations while making a handover, transmits to the base stations using the same spreading code, and in which the signals received by the base stations are combined in a base station controller or the like.
CDMA is a multiple access method, which is based on the spread spectrum technique, and which has been applied only recently in cellular radio systems, in addition to the prior FDMA and TDMA methods. CDMA has several advantages over the prior methods, for example spectral efficiency and the simplicity of frequency planning.
In CDMA, each signal consists of an individual pseudorandom sequence which modulates the base frequency, simultaneously spreading the band of the data signal. The data signals of several system users are transmitted simultaneously on the same frequency band. The users are distinguished from one another on the basis of the pseudorandom sequence, called the spreading code. Correlators provided in the receivers are synchronized with a desired signal, which they recognize on the basis of the spreading code, and they restore the band of the signal to its original band width. Signals arriving at the receiver and containing the wrong spreading code do not correlate in an ideal case, but retain their wide band and appear thus as noise in the receivers. The spreading codes used by the system are preferably selected in such a way that they are mutually orthogonal, i.e. they do not correlate with each other.
In a typical cellular radio environment, the signals between a base station and a mobile station propagate along several paths between the transmitter and the receiver. This multipath propagation is mainly due to the reflections of the signal from the surrounding surfaces. Signals which have propagated along different paths arrive at the receiver at different times due to their different transmission delays.
The receiver generally utilized in a CDMA system is a so-called rake receiver, which consists of one or more rake branches, i.e. correlators. The rake branches are independent receiving units, the function of each unit being to combine and demodulate one multipath-propagated received signal component. The realization of a rake branch is described in greater detail in Modern Communications And Spread Spectrum (by G. Cooper and C. McGillem, McGraw-Hill, New York, 1986, Chapter 12). In addition to the rake branches intended for receiving signals, a CDMA receiver typically comprises at least one separate searcher branch, the function of which is to search for the different signal components of the signal transmitted with the desired spreading code and to detect their phases. Each rake branch can be made to correlate with a signal component which has propagated along an individual path, and each signal component arrives at the receiver with a different delay. The rake branches are controlled by giving the correlator information about the desired spreading code and its phase. Since the signal components propagate along separate paths, they also attenuate often independently. In a conventional CDMA receiver the signals of several correlators are preferably combined, whereupon a signal of good quality is achieved despite the multipath propagation along the radio path.